So I tested it with Amazon SES relay and then with a different SMTP server (PurelyMail's). It works with PurelyMail's but not with Amazon SES. The error message refers to the '<' character.
So now, looking at emails sent to me using the working SMTP server, I am noticing that Commento's app does something different with the headers:
Here's a typical test email header sent from the Cloudron email settings page:
From: Cloudron <email@example.com> To: firstname.lastname@example.org
Here's one sent from Commento's app. Note that this is the one that I received via PurelyMail's SMTP server. This would have failed with Amazon SES:
From: Commento <<email@example.com>> To: Joe Smith <<firstname.lastname@example.org>>
I swapped out the domain to mydomain.com. But notice that Commento has the extra brackets, and whereas PurelyMail's SMTP server handles it fine, Amazon SES chokes on it. Is this something in Cloudron's implementation of Commento that adds this?
Thanks for taking a look!